It is well known to employ permeable membranes to separate or selectively enrich a gas mixture. It is also generally known that gas separation membranes may be cast from polymers. The separation of gas components by polymer membranes is thought to depend on chemical affinities, kinetics and structural characteristics; it being known generally that rubbery polymers are characterized by high diffusion and relatively low selectivity while glassy polymers are characterized by lower diffusion and higher selectivities.
Polymer blending has traditionally been thought to be either problematic or of no benefit in the membrane field, primarily because different polymers are generally not miscible with one another. Those few polymers which are thought to be miscible offer no blending advantage in the membrane field because of various reasons, including difficulty in blending, poor mechanical properties, limited range of gas transport properties, and complex relationships between blend composition and gas transport properties.
U.S. Pat. No. 5,055,116 describes a blend of aromatic polyimides, in which the blending of the polymer components is adjusted to achieve certain permeability and selectivity of a polymer membrane. The final properties of a new polymer membrane may be predicted so that a membrane with those desired final properties can then be manufactured.
U.S. Pat. No. 5,055,116 indicates that the gas transport properties of the membrane prepared from the polyimide blends are predictable and the membrane may be "engineered" to achieve the desired final properties. To the contrary, the gas transport properties of the present invention are unpredictable and surprisingly good.